In many electrical systems with a voltage source which, for example, is a voltage source for an electrical power supply system, a series circuit is also provided which is connected to the voltage source via a conductor and has an inductance and a capacitance, with the inductance then being connected in series with the capacitance via a conductor. An active or passive electrical load or a current source, such as a rotating electrical machine with an associated power-electronic feed device, can be connected in parallel with the capacitance, in which case the rotating electrical machine can normally be operated as a motor or generator.
If the voltage source is a voltage source of an electrical AC or DC voltage power supply system, there can be stringent requirements relating to the feeding in of undesirable alternating currents. Undesirable alternating currents can originate, for example, from the load side of the electrical system, that is to say by way of example from a rotating electrical machine and its power-electronic feed device. The dimensioning of the inductance and the capacitance can be used to restrict undesirable alternating currents. However, the undesirable alternating currents can be reduced only to limited restricted extent by the dimensioning of the inductance and the capacitance, and this is associated with increased weight, spatial requirements and additional costs.
An apparatus such as this is disclosed in US 2005/0083627 A1. In this document, the disclosed apparatus has a filter circuit which is connected via a conductor to a voltage source for an electrical system. The filter circuit is formed by two parallel-connected series circuits, each having an inductance with a capacitance. A transformer winding of a coupling transformer is connected in series with the filter circuit.